Japanese knotweed identification is not always easy, and yet to ignore a potential infestation could have very destructive and costly consequences.
The best time for japanese knotweed identification is during mid-summer and early autumn. During spring, reddish/purple shoots appear from the ground. These can grow up to 2cms a day, thus rapidly forming dense stands of bamboo-like stems that develop green heart- or shield-shaped leaves.
By early summer the mature Japanese knotweed stems are hollow with purple speckles and can reach up to 3 meters in height. The leaves alternate on each side of the stem producing an obvious knotweed zigzag pattern.
The flowers that emerge by late summer are creamy-white and appear in lengthy cluster/spike formations. Japanese knotweed spreads mainly from its underground rhizomes/roots which lie dormant, but alive, over the winter months.
The rhizomes can spread 7 meters outwards and 3 meters deep from the visible, aboveground stems. It is therefore effortless to accidentally fragment pieces of rhizome and spread them by disturbing the soil several meters from where the stems appear. As new growth from seeds is very rare, it is the testament to the plant’s incredible invasiveness that it has spread to most parts of the UK (and Western Europe and North America, for that matter) by merely fragmenting and translocation rhizomes in contaminated soil.
Our identification guide below should help you identify Japanese knotweed from its various features. We also provide a free service to review photographs of your suspected knotweed plants.
From experience of many hundreds of photos sent to us, it is quite likely that you don’t have knotweed at all and instead have one of the plants listed on our page of ‘usual suspects.’ If you do have knotweed, then please contact us today to speak to one of our expert consultants to see how we can help.
• New leaves are dark red and 1 to 4cm long
• Young leaves are green and rolled back with dark red veins
• The mature leaves are green and heart-shaped but flattened at the base (a bit like a shield) and are usually around 12cm long
• Mature growth forms dense thickets 2m to 3m tall
• Stems are ‘bamboo-like,’ with obvious nodes/rings and purple speckles
• Leaves shoot from the stem nodes alternately, which creates a ‘zig-zag’ pattern at the top